the alter command conflicted with the check constraint (but no rows violate constraint)

Question

I am trying to add a check constraint on sql server 2008. I am getting the following error when i run this code:

alter table db.dbo.myTable add constraint
noWWWifProgramIsKnown check
(dbo.checkcountuniversalservice(A, B,
C)=0 and  program='WWW' )

raises error:

The ALTER TABLE statement conflicted with the CHECK constraint "noWWWifProgramIsKnown". The conflict occurred in database "db", table "dbo.tableuniversalservice".

Marc_s' answer on SO says that this is likely b/c some row violates the constraint: The ALTER TABLE statement conflicted

However, no rows violate the constraint because this returns 0:

select COUNT(*) from db.dbo.tableuniversalservice 
where
(dbo.checkcountuniversalservice(A, B, C)=0
and 
program='WWW'
)

What am I missing?

Answer 1

I think your check is backwards... in order for the constraint to pass, it should evaluate to TRUE or to UNKNOWN. Your SELECT statement is telling you that no rows meet the constraint instead of telling you that no rows violate the constraint.

From MSDN:

CHECK constraints reject values that evaluate to FALSE.